Area Forecast Discussion
Issued by NWS State College, PA

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853 FXUS61 KCTP 211549 AFDCTP Area Forecast Discussion National Weather Service State College PA 1049 AM EST Tue Jan 21 2020 .SYNOPSIS... Northwest flow aloft and a large area of surface high pressure building southeast from the Upper Mississippi Valley will bring a lengthy string of dry days for the remainder of this week. Temperatures will still be up to several degrees below normal today before moderating back to normal on Wednesday, then above normal to close out the work week. A slow-moving area of low pressure will bring the potential for widespread and significant snow or mixed precipitation this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Still a few flurries within the stratus over the NW and W Central Mountains at mid morning, but should not amount to much. Dry air entrainment is producing a clear slot from the NC to the Central Mountains this morning which will fill in again this afternoon. M/C with chance for flurries will remain across the Northern and Western Mtns while clear to partly cloudy skies persist across the SE zones. Temps will be mainly in the 10-15F range through shortly after sunrise this morning with some single digits in the deeper valleys. Winds will be from the northwest and average mainly 5-10 mph with some gusts into the low-mid teens. && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/... Upper ridge and elongated surface high will build east from the Ohio Valley and become located across PA on Wednesday. This weather feature will provide abundant sunshine and light wind for Wednesday, along with slightly milder temps. Highs Wednesday will be a few degrees higher compared to Tuesday, ranging from the mid to low 30s north to the upper 30s in the Lower Susq Valley. Fair weather and moderating temperatures are expected Wed and Thu, as high pressure settles along the east coast. && .LONG TERM /FRIDAY THROUGH MONDAY/... Focus in the medium range continues to be on the increasing probability for another weekend winter storm, though there are some distinct differences with this storm compared to this past Saturday`s event. T the main feature that sticks out is the presence and favorable track of a distinct and fairly deep upper low across Central or Southern PA, this track implies a one-two punch of warm conveyor, then cold conveyor-belt precip. Ptype will be an issue across the SE third to perhaps half of our CWA, while parts of the Central Mtns and most or all of the Northern Mtns should be cold enough for all snow. Furthermore, FGEN banding and briefly heavy snow rates are possible late Friday Night/Sat morning as the mid-upper low center passes and max UVVEL intersects the descending, favorable thermal band for dendritic snow growth. Max wet bulb AOA 925 mb is about 1-2C along and to the SE of Interstate 81/I-78 corridor in Southern PA, while it`s AOB zero further north and west. Sfc temps should range from the L30s across the north, mid 30s in central PA, and Mid to Upper 30s in the SE. This vertical thermal structure and the primary nose of the warm layer aloft (and PWAT above 20mm) staying just below PA will likely result in a wet snowfall across Central PA and portions of southern PA (between and after a few to several hour change to sleet. The resulting anomalous southeasterly low level jet and plume of Atlantic moisture overrunning a dome of cold/stable air east of the Appalachians justifies increasing POPs Saturday to 90-100 percent at this day 3.5 - 4 time range. The latest GEFS mean qpf ranges from 0.6 to 1 inch across the forecast area by Saturday evening. GFS forecast soundings imply mostly snow even as far south as the greater Harrisburg area. Despite the high confidence of significant precipitation, plenty of uncertainty still remains with regard to ptypes across the area based on slightly different sfc and upper low tracks. The primary surface low is progged to weaken over the Ohio Valley Saturday, as secondary low deepens and tracks up the Mid Atlantic coast in classic Miller B fashion. Quicker development of the coastal low would imply low level winds backing from SE to NE, supplying cold air at low levels during the storm on Saturday. This would mean more wintry precip and less rain. Quicker development of the coastal low could also translate to better FGEN forcing/banding on the NW side of the developing low, which would translate to heavier precip and some dynamic cooling. It will be a couple of days before we have these answers. As always, it will be interesting to see how the hi-res guidance handles these features as the event comes in range. Based on the latest ECMWF ensemble 850temps and GEFS plumes, current forecast is for a wet snow NW half of the CWA and a wintry mix across much of the remainder of the forecast area Friday night into Saturday. A deep, moist northwest flow associated with deformation band should result in lingering light upslope snow/rain showers Sat night into part of Monday, primarily over the Allegheny Plateau. Do not expect any contribution from the lakes, as the air mass behind departing storm looks too mild to support lake effect. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/... MVFR cigs to continue most of the day for KBFD and through mid afternoon at KJST because of lake induced clouds off of Lake Erie. Elsewhere, VFR cigs will persist through the forecast period. .Outlook... Wed-Thu...Mainly dry. Fri-Sat...Snow/wintry mix Fri night and Saturday. Restrictions likely. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...DeVoir/Lambert SHORT TERM...Lambert LONG TERM...Fitzgerald/Lambert/Colbert AVIATION...Gartner/Banghoff

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